The present invention pertains to a process for separating ethylene oxide from aldehyde-type impurities; formaldehyde as well as acetaldehyde.
Ethylene oxide accompanied by the said impurities essentially results or derives from the production of ethylene oxide by gas-phase catalytic oxidation of ethylene by molecular oxygen.
The ethylene oxide is isolated from the gaseous mixture formed in the zone of catalytic oxidation of ethylene in a known manner, usually in several steps comprising:
(a) absorption by water, consisting of bringing the said gaseous mixture into contact with water to obtain an aqueous solution of ethylene oxide usually containing about 2-3 wt.% ethylene oxide, aldehyde-type impurities and, in the dissolved state, normally gaseous compounds, especially CO.sub.2,
(b) desorption, consisting of subjecting the dilute ethylene oxide solution obtained in (a) to steam distillation to obtain a gaseous mixture containing CO.sub.2 and aldehyde-type impurities, formaldehyde and acetaldehyde, in addition to ethylene oxide, which is usually present in amounts of 30-60 wt.%,
(c) reabsorption by water of the ethylene oxide contained in the gaseous mixture obtained in (b) to form an 25 aqueous ethylene oxide solution usually containing about 5-25 wt.% ethylene oxide and also CO.sub.2, as well as aldehyde-type impurities, and
(d) distillation of the aqueous ethylene oxide solution obtained in (c) to obtain ethylene oxide with little or practically no water, on the one hand, and an aqueous stream practically free from ethylene oxide, on the other hand.
The ethylene oxide thus obtained in (d) contains a level of aldehyde-type impurities, formaldehyde and acetaldehyde, that is so high that it is not possible to reach the quality required industrially.
U.S. Pat. No. 4,134,797 proposes a process for reducing the aldehyde-type impurity content in ethylene oxide, according to which an aqueous stream essentially free from ethylene oxide is drawn off at the bottom of the column used. This depletion of ethylene oxide in the said aqueous stream is guaranteed by the introduction of steam as a distilling fluid into the column. The ethylene oxide leaves the column simultaneously at three different levels in the form of a stream of ethylene oxide rich in formaldehyde, a stream rich in acetaldehyde and finally a stream of ethylene oxide that may contain about 0.001-0.035% aldehyde-type impurities. This latter stream is obtained only as a result of a very complex distillation process which leaves about 25-40% of the ethylene oxide in the nonpurified form according to the examples described.